This project aims eventually to contribute to an understanding of the genetic mechanisms underlying the establishment of the neural circuitry, related to the production of species-specific vocalizations. The current research investigates the role of inherited factors in determining the species-specific attributes and normal development of primate vocalizations. A principal goal of this work is to establish the inheritance patterns for the isolation call in squirrel monkeys, by crossbreeding individuals with different vocal phenotypes. The subjects in question demonstrate these phenotypic differences from birth, and show karyotypic differences, as well. The present studies utilize quantitative procedures modified for this project to determine if isolation call phenotype in the hybrid offspring match one or the other parental type, or form additional hybrid categories of calls. This approach has been applied to the isolation calls of 15 newborn hybrids. The results show that hybrids produce a range of phenotypes, some matching one or the other parent, while others form a new phenotype, intermediate to the parental calls. The extent to which the hybrid calls match those of the parents depends on the subspecies of the parents. In related studies investigating the production of other subspecifically distinct vocalizations by male hybrid squirrel monkeys, vocalizations produced during ritualized social displays distinct to each of two parental subspecies have been found in the hybrid male offspring. Observations of one male hybrid raised apart from other conspecifics beginning shortly after birth indicate that displays and associated vocalizations distinctive to both parental subspecies can be produced by an individual in the absence of a natural model.